Since the 1950-ies/1960-ies, air supply to divers that use a hose is no longer established by hand pumps. Divers get their air from a battery of compressed air tanks. The air goes through a control-valve to a control valve panel (see picture) from which, in this case two divers can be supplied with the sufficient amount of air.
A high pressure compressor makes sure that the battery of air tanks is constantly filled under an equal pressure. In case the compressor shows problems, the air in the tanks is enough to ensure a safe decent for the diver(s).
The latest in control panel technology. Courtesy of Syrotechnik Germany.
|Another important device for safety is the dive-regulator. It is a combination of a reducing-valve and a control-valve that automatically supplies the diver with the sufficient amount of air in every depth. Lets say the dive-regulator is set to give 50 liters a minute, it will then automatically give 100 liters a minute when the diver is in 10 meters depth. The risk of "falling" underwater is minimized.|
Visit the "Going deep" section in the historical diving section to learn all about the pioneers of mixed gas diving.
To supply the diver with a breathing mixture there are 3 general ways:
The half open system
Similar to the heavyweight helmets described in the historical section military and commercial mixed gas helmets and masks are provided with a backpack arrangement that carries a CO2 absorbent canister and a venturi circulating pump. The appropriate mixture is supplied through a hose from the surface and enters the venturi pump. It passes through the absorbent canister and then circulates the helmet. Nearly all the gas is re-inhaled. This cuts down on the costs. The systems feature a bailout system: one or more small cylinders on the back containing the breathing mixture. As soon as the diver has a problem with the surface supplied gas, he can switch to the bailout system and finish the dive in a safe way.
The open system
The open system is very similar to the conventional supply of compressed air. The mixture comes down to the helmet in a continuous flow. Exhaled gas leaves the helmet through the exhaust valve. This system is a lot cheaper to buy but uses a lot more gas! A backpack arrangement is not needed. Many divers prefer this system over the half open system. In their opinion you can buy a lot of gas instead of the backpack.
The SCUBA system
Self Contained Underwater Breathing Apparatus. The diver carries the mixture in containers on his back. After inhaling through the regulator that is attached to the mask the exhaled air leaves the regulator. The diver now has complete freedom and has a chance to stay down longer than using compressed air. There is however no proper surface communication.
When diving with compressed air three major problems may occur at greater depth. These are:
This is explained in the "Going Deep" section in the historical diving section. In that section you can read that the US Navy played in important role in mixed gas diving research.
American professor Elihu Thomson suggested in 1919 to use Helium as a substitute for nitrogen in the breathing mixture. Helium is an inert (will not combine with anything) gas that has absolutely no narcotic influence on the diver. Therefore, when using a breathing mixture of helium and oxygen, the depth of a dive is no longer limited by nitrogen narcosis effects. Today, this mixture called heliox is used up to depths of 600 meters without any serious cases of narcosis.
Helium is also a very light gas. Absorption and dissolution out of the body tissues will go much faster than with breathing gas containing nitrogen. This means that less decompression time is needed using heliox.
On deep dives the amount of oxygen has to be carefully controlled. Helium prevents narcosis but does not protect the diver from oxygen poisoning. At any given depth a minimum amount of oxygen is needed to keep the diver alive and a maximum amount that the diver can tolerate to avoid oxygen poisoning. The mixture is created for that specific dive and pumped down to the diver.
Disadvantages in using helium
There are 3 disadvantages in using helium:
In Europe the development of deep
diving techniques was very slow. One of the consequenses of world war II was
that helium was hardly available and nobody could afford it.
For this reason they were forced to experiment with other inert gasses.
A Swedish guy named Arne Zetterström discovered that hydrogen could be used in a breathing mixture instead of helium. Hydrogen is not hard to get and it is cheap. However, there was a big problem in mixing it with oxygen. As soon as the oxygen part in the mixture is over 4 % it becomes highly explosive ! With a percentage less than 4 % it can not be used as a breathing mixture (you would pass out due to a lack of oxygen).
Zetterström found the solution to this problem: In 40 meters deep the mixture with 4 % oxygen is enough to live: the partial pressure on the body and the mixture is (of course) higher than it is on the surface. In the tests that followed around 1945 he used normal compressed air to a depth of 40 meters and that switched over to the heliox mixture. He reached a depth of 160 meters without any problem. This was deeper than divers ever came on heliox in those days !
On coming up at 40 meters depth he switched back to normal compressed air and surfaced without a problem. In 1945 Zetterström died during a Hydrox dive. It was not the mixture that killed him but his surcace team: they pulled him up too quickly, there was no time to switch to compressed air and when he surfaced it turned out he had passed out due to a lack of oxygen and drowned.
It took at least till the 1960-ies for scientists from the French COMEX company to pay attention again to Zetterströms ideas. Divers reached depths up to 240 meters in the tank without problems. In depths of 300 meters divers felt narcotic symptoms. After adding a small percentage helium to the mixtures the problems soon went. In the end of "Hydra" program as it was called, divers went down to 701 meters !
These days, diving on hydrox is not often done any longer. It is expensive and deep dives are very risky. Now, many times ROV's (Remote Operated Vehicles) are put to work in such jobs.
The mixture described above is called hydro-heliox. It consists of 1% oxygen, 49 % hydrogen and 50% helium
In 1980 two English dive physiologists
reached a depth of 660 meters in a tank. The divers stayed there
for 36 hours and decompressed in 35 days.
Soon after that the Americans reached 686 meters in the tank. In both cases divers were breathing Trimix (10% oxygen, 10% nitrogen, 80% helium).
High Pressure Nervous Syndrome (HPNS)
In 1956 a Navy diver went down to a depth of 180 meters on a mixture of oxygen and helium. He said he had a dizzy feeling in his head and a weird feeling in arms and legs. In later tests to even greater depths the diver's hands shook and he was ill. There were even situations called "microsleep". For a short period of time the diver is completely blocked off like he is asleep. Nobody knew what it was but that it had something to do with the helium. Many tests with different breathing mixtures as described in this page were carried out since than to find a way to prevent HPNS.